Today, on irc, someone asked interesting question.
Basically she ran a query like:
select a, b, c, d, e, f from table order by a
then, she processed the query to get, for each a array of unique values of b, c, d, ...

On 23rd of December, Tom Lane committed patch:
Support ordered-set (WITHIN GROUP) aggregates.
This patch introduces generic support for ordered-set and hypothetical-set
aggregate functions, as well as implementations of the instances defined in
SQL:2008 (percentile_cont(), percentile_disc(), rank(), dense_rank(),
percent_rank(), cume_dist()). We also added mode() ...